I. Energy Storage
There is an increasing demand for wearable/portable electronics leading to an increased need for suitable electrical storage devices to power them. For this application, the ideal energy storage device would be safe, biodegradable and flexible with a high capacity and a discharge at a high rate to support peak consumption. Electrical storage can take place in batteries or capacitors. For batteries, electric energy is stored through chemical redox reactions in accordance with long-term small current discharge. In general, batteries provide higher energy power for storage but they are temperature sensitive and have limited cycle life and long charging time. Furthermore, batteries are unsafe because they contain flammable electrolyte(s) and are pressurized. This is particularly concerning when considering “wearable” applications as battery materials are brittle and rigid. For capacitors, electric energy is stored through physical charge accumulation. Capacitors typically contain low toxicity materials and no corrosive electrolytes and they have a long cycle life, a greater power density than batteries with rapid charge and discharge capabilities operating in a wide temperature range. Because of these properties, capacitors have applications in multiple fields such as energy storage, digital memory, power conditioning, suppression and coupling, motor starter, sensing, and the like. However, capacitors present some limitations such as low energy density (low amount of energy storage) that need to be addressed. Additional limitations stem from the fact that materials and processes utilized to generate a capacitor with improved performance typically result in bulky, large size, and heavy weight capacitors.
Therefore, there is a need for an energy storage composite for efficient energy storage. Furthermore, composites prepared with mechanically flexible, lightweight and biodegradable/renewable materials are desirable.
II. Conductive Polymers:
Conductive polymers are the basis of organic electronics or polymer electronics as opposed to electronics based on inorganic conductors and semiconductors such as copper and silicon. Conductive polymers induce conductivity as a result of the formation of charge carriers upon oxidation or reduction of their conjugated backbone. Simply put, conductive polymers are kind of glue that holds electrode materials together while shuttling electrons within and between the intra-molecular chains. Some of the attractive properties useful in a sustainable process of conductive polymers are their biodegradability, low volatility, low toxicity, and thermal stability. Furthermore, conductive polymers may be somewhat flexible as they can be deposited on flexible materials/substrates such as plastic and more recently paper and textiles.
Major drawbacks of conductive polymers include their lack of stability in their conductive state leading to a relatively low working voltage and their structural limitations. Additional disadvantages are low capacitance, strength, tensility, porosity, surface area, and rate capability. Some improvement of a few of these properties have been achieved thus far for limited applications by using complex processes including hazardous substrates such as carbon nanotubes and graphene oxide.
III. PEDOT:PSS:
Amongst various organic materials used for organic electronics is a conductive polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)). PEDOT:PSS consists of two oppositely charged polymers: poly(3,4-ethylenedioxythiophene) (PEDOT) that is positively charged whereas poly(styrenesulfonate) (PSS) is negatively charged. Together, PEDOT:PSS makes a macromolecular salt available as aqueous dispersions that can reach high conductive values up to 1000 Siemens/cm. PSS acts as a polyanionic to neutralize the highly oxidized PEDOT chains. The PEDOT molecule can lose one or more electrons whereas the PSS receives them.
PEDOT:PSS belongs to the class of mixed ionic-electronic conductors (MIECs) that have significant conduction both ionically and electronically. MIECS are able to capacitate rapid solid state reactions and are widely used in devices for energy conversion and storage. Under an externally applied electrical field, the charged PEDOT and PSS polymer chains will move in opposite directions so that the material will be electrically polarized and the capacitor becomes charged. After removal of the applied electrical field the ions will move back to their original position so that the material loses its polarisation.
As such, PEDOT:PSS exhibits some of the desired properties to design a novel energy storage that would be lightweight, flexible, and renewable. However, PEDOT:PSS presents numerous key limitations such as poor mechanical stability (i.e. cracks and discontinuities), limited capacitance, and relatively low charge/discharge. Strategies implemented to attempt to enhance conductive polymers' properties designed for various applications such as electroluminescent devices, solar cells, and electrode materials include the use of secondary dopants and the coating or printing of the conductive polymers on paper or on fabric to generate a conductive composite film. Drawbacks of existing composites and methods are that these composite films are typically prepared at high temperature and intermixed with hazardous chemicals such as copper, indium oxide, and graphene oxide and intermixed with complex structures such as nanowires, nanoparticles, and carbon nanotubes, using complex protocols involving multiple steps such as spin coating, layer-by-layer deposit, printing, etching, spray film formation, and patterning of complex interdigitated structures of different thin film materials inside a bulk volume with a high surface area or by including a carbon or copper foam. These methods also include the formation of a thin film with loss of capacitance as the film thickness increases, thus becoming unsuitable for most electronics applications. Furthermore, under certain circumstances, readily flammable or toxic solvents must also be employed.
Thus, there is a need for a simple method to create such an energy storage composite using safe and biodegradable materials that are conceivably available in large quantities.
IV. Glucose Polymers as Structural Materials:
Cellulose is one of the most abundant polymers produced naturally by numerous organisms including trees, algae, bacteria, and fungi. Cellulose is a linear polymer structured as a fibrous network. There are two major building blocks that can be derived from cellulose: regular fiber with micrometer size diameter and nanocellulose fibers with nanometer size diameter. Cotton fibers almost entirely consist of cellulose whereas wood contains almost 50% cellulose.
Cellulose consists of glucose units with hydrogen bonding between the hydrogen of hydroxyl groups and oxygen holds the cellulose fiber tightly together designed to maximize the stability and durability of plants. This property allows for the formation of large porosity that in turn allows for fast access of ionic species to the electrode surfaces. As such, it can support the redox chemistry. The three-dimensional hierarchical structures formed by cellulose fibers combined with the capability of incorporating the functionality of other materials, make cellulose an interesting material for applications in electrical and electrochemical devices. Cellulose in the form of paper has been used as material as the basis for the production of simple integrated electric and/or electronic circuits. Conductive components can be, for instance, deposited by means of ink-jet printing on the surface of paper. This method of printing minimizes penetration of the conductive polymer and prevents its further deep penetration into the fiber. Therefore, the use of cellulose in the form of paper may not be the optimal form to fully take advantage of the beneficial structural properties that cellulose may offer.
Starch is a polymeric carbohydrate consisting of the same glucose-based repeat units than cellulose but with different groups. It can be derived from potatoes, wheat, corn, rice, cassava, etc. Starch is one of the most commonly used biopolymers in industries because of nontoxicity, biodegradability, biocompatibility, low cost, and being renewable and abundantly available in nature. Similar to cellulose, it can support redox chemistry and can thus be a candidate to improve the structural deficiencies of conductive polymers.
V. Secondary Dopants:
In an effort to improve conductive polymers' properties, secondary dopants may be applied in the form of a liquid and/or vapor to conductive polymers already doped with a primary dopant. Secondary dopants induce significant changes in molecular conformation and dependent properties such as solution viscosity, Vis-UV spectra, crystallinity, dielectric constant, conductivity/temperature relationship, and the like. These effects may persist after complete removal of the secondary dopant. For instance, the conductive polymer PEDOT:PSS has been reported to form entangled structures with the primary dopant PSS. The degree of disorder, or amorphous phase, may vary from 10% to 100% and the phase separation has been reported to proportionally affect the properties of the conductive polymer. The phase separation state can be controlled in some instances by addition of “secondary dopants” that can facilitate inter-molecular charge transfer.
Secondary dopants such as polyols (e.g. glycerol), polyethers (e.g. polyethylene glycol (PEG)), solvents (e.g., organosulfur solvents such as dimethylsulfoxide (DMSO)) have been utilized with the goal of increasing conductivity and plasticity of conductive polymers. For instance, glycerol is known to improve plasticity and water absorption allowing for ion movement. However, incorporation of secondary dopants has to be judiciously designed (e.g. ratio, sequence of addition into the mixture, etc.) as it may result in significant and detrimental alterations of the properties of conductive polymers such as reduction of tensility, embrittlement, scission, phase separation, etc., of the resulting material.
VI. Need for Flexible, Lightweight, Biodegradable Energy Storage
The availability of a composite that is biodegradable, flexible, lightweight while generating higher performance differentials would represent a major advance in the field of electronics in general, and the field of wearable and embeddable devices in particular. Hence, there is still a need to develop an energy storage composite that may partially or wholly be made of renewable and/or biodegradable material. The present invention satisfies these needs and provides related advantages as well.
Although the polymers and the components described here offer partial and limited advantages when considered separately, the outcome of their combination to achieve the structural and functional properties necessary to yield a type of energy storage composites described in the present invention, remains unanticipated. Indeed, only some of the combinations described in the present invention resulted in superior composites whereas some combinations reported here resulted in non-functional and/or structurally challenged mixes with no clear applications.
The present invention provides the combinations of conductive polymers which have been enhanced with specific secondary dopants and polymeric structural materials that have surprisingly yielded the desirable properties. Ideally, the components used for the preparation of an energy storage composite would be flexible, lightweight, biodegradable and/or renewable, available in large quantities, and would be easily processed. Furthermore, their combination would give rise to an efficient composite with desirable energy storage properties and that favors long term stability and minimize device failure.